The invention relates to a sensor arrangement in an electromagnetic rotary drive which is designed as a bearing-free motor with a magnetically journalled, permanent magnetic rotor and a stator and to a method for the operation of a rotary drive of this kind.
A bearing-free motor with a permanent magnetic rotor is an electromagnetic rotary drive which comprises a permanent magnetically excited rotor and a stator, with the rotor being journalled without contact by means of magnetic forces. The characteristic to which the bearing-free motor owes its name is that it has no separate magnetic bearing for the rotor. For this the stator is designed as a bearing and drive stator and the rotor as a passive magnetic rotor which serves both as a bearing rotor and as a drive rotor. The stator is designed or provided with electrical windings respectively in such a manner that it produces an electromagnetic rotary field which exerts, on the one hand, a torque on the rotor which drives its rotation about the axis of rotation and which, on the other hand, exerts a transverse force on the rotor which can be set in any manner desired so that its radial position with respect to a plane perpendicular to the axis of rotation can be predetermined or actively controlled respectively. Thus in the operating state the rotor can be actively controlled and driven respectively by means of the electric windings of the stator with respect to three degrees of freedom, namely the rotation about the axis of rotation and the radial position in the plane perpendicular to the axis of rotation (two degrees of freedom).
With respect to three further degrees of freedom, namely tiltings with respect to the plane perpendicular to the axis of rotation (two degrees of freedom) and the axial position, the rotor is passively magnetically stabilized, that is, not in a controllable manner, by reluctance forces. Thus in the operating state the rotor can be both driven and journalled without contact through the magnetic interaction between the bearing/drive stator and the rotor without separate magnetic bearings being present for this. The term xe2x80x9cbearing-free motor with a permanent magnetic rotorxe2x80x9d is to be understood in this sense for the following explanations. With respect to further details of the design and of the excitation and regulation respectively of the bearing-free motor, reference is made here to Schoeb et al. U.S. Pat. No. 6,100,618 entitled Rotary Machine with an Electromagnetic Rotary Drive issued Aug. 8, 2000.
In Schoeb et al. U.S. Pat. No. 6,100,618 entitled Rotary Machine with an Electromagnetic Rotary Drive issued Aug. 8, 2000 a bearing-free motor of this kind is disclosed in the example of a rotation pump. In the latter the rotor of the bearing-free motor is provided with vanes and thus forms an integral rotor, which means that it takes over the function of the rotor of the pump in addition to the function of the rotor of the electric motor. Pumps of this kind are advantageous in particular for those uses in which the fluid to be forwarded must not be contaminated, for example for the forwarding of biological liquids such as blood or highly pure liquids such as purest water. In addition rotation pumps of this kind are suitable for the forwarding of aggressive liquids which would destroy mechanical bearings in a short time.
In comparison with conventional pumps with a magnetically journalled rotor, pumps of this kind, which operate in accordance with the principle of the bearing-free motor, have the advantage of being extremely compact and space-saving and nevertheless having all the advantages of the non-contact magnetic journalling of the rotor even at high performance or forwarding power respectively. This is one of the reasons why pumps of this kind are suitable among other things as blood pumps for uses inside and outside the body.
For the operation of a bearing-free motor with a permanent magnetically excited rotor, in particular for the regulation of the drive and the position of the rotor, which usually takes place by means of a vector regulation method or a field-oriented regulation method respectively, it is necessary to know the direction of the rotor magnetization, that is, the momentary position of the magnetization of the rotor relative to the stationary stator system. In accordance with Schoeb et al. U.S. Pat. No. 6,100,618 entitled Rotary Machine with an Electromagnetic Rotary Drive issued Aug. 8, 2000 it is proposed for this to arrange four magnetic field probes in the air gap between the stator and the rotor.
Even though this arrangement has proved its worth in practice, difficulties nevertheless result for several forms of the rotor magnetization. If for example the rotor is magnetized in a block pattern, then the signal measured in the air gap by the magnetic field sensors varies only very littlexe2x80x94if at allxe2x80x94over a relatively large rotational angle of the rotor so that a unique determination of the momentary direction of the rotor magnetization is practically no longer possible. In such cases separate rotation sensors would then have to be provided.
One object of the invention is thus to propose a better sensor arrangement in a bearing-free motor with a permanent magnetically excited rotor which enables the direction of the rotor magnetization and thus also the geometrical rotor angle to be determined. In accordance with a further aspect the axial position of the rotor is also to be determinable by means of the sensor arrangement. It is furthermore an object of the invention to propose a corresponding method for the operation of a bearing-free motor with a permanent magnetic rotor.
In accordance with the invention a sensor arrangement in an electromagnetic rotary drive is thus proposed which is designed as a bearing-free motor with a magnetically journalled, permanent magnetic rotor and a stator, said sensor arrangement serving for the determination of the direction of the rotor magnetization and/or of the axial position of the rotor and comprising at least two sensors for the determination of a magnetic flux. The two sensors are arranged with respect to the rotor in such a manner that partial fluxes of that magnetic flux which the stray field of the permanent magnetic rotor produces can be determined with them.
In accordance with the invention the measurements for the determination of the position of the rotor magnetization and/or of the axial position of the rotor thus take place in the stray field of the rotor. Through this the direction of the rotor magnetization with respect to the stationary stator systemxe2x80x94and thereby also the geometric angular position of the rotorxe2x80x94can also be determined in those cases in which the rotor magnetization is not continuously distributed over the rotor, for example in a block-patterned rotor magnetization. With the arrangement in accordance with the invention and the method in accordance with the invention respectively the momentary position and the direction of the rotor magnetization respectively can be reliably determined during the operation of the motor for all rotor magnetizations which are relevant in practice. Since in addition no separate rotation sensor, that is, one that is different from the actual rotor, is required for this, the sensor arrangement in accordance with the invention is particularly simple in regard to the apparatus and in particular also in regard to the mounting technology and is thereby economical.
A further advantage lies in that the axial position of the rotor with respect to the stator can be determined with the arrangement in accordance with the invention or the method in accordance with the invention respectively without further sensors being required for this. This also reduces the cost and complexity of the apparatus for the bearing-free motor, especially the number of sensors required, because two measurement values which are required for the operation, namely the direction of the rotor magnetization and the axial position of the rotor, can be measured by means of a single sensor system.
The sensors are preferably arranged in such a manner that the partial fluxes of the magnetic flux can be determined in a first measurement direction and in a second measurement direction, with the measurement directions in each case extending perpendicular with respect to the axial direction which is defined by the desired axis of rotation of the rotor, because the evaluation of the sensor signals is thereby simplified.
The rotor of the bearing-free motor is preferably designed in a disc or ring shape. The sensors are then arranged with a displacement from the rotor with respect to the axial direction, which means in particular above or below the rotor with respect to its desired axis of rotation. This is a particularly simple arrangement in order to measure in the axial stray field of the rotor. Naturally other geometries of the rotor, for example also bell-shaped ones, are also possible.
Independently of the special geometry of the rotor it is however advantageous to arrange the sensors outside the space which is located between the rotor and the stator. Thereby, namely, no sensors are required in the air gap between the stator and the rotor so that this air gap can be designed smaller, which has a very positive effect on the magnetic coupling between the stator and the rotor.
A further advantageous measure consists in arranging the sensors in a common measurement plane which extends perpendicular to the axial direction, because the evaluation of the sensor signals is further simplified through this.
The sensors are preferably arranged in such a manner that the first and the second measurement direction extend at an angle with respect to one another which is approximately 90xc2x0 or is unequal to an integral multiple of the quotient of 180xc2x0 divided by the number of pole pairs of the rotor. In regard to as good a signal measurement and evaluation as possible it has proved useful in practice to arrange the sensors in such a manner that the first and the second measurement direction extend at an angle to one another which is equal to the quotient of 90xc2x0 divided by the number of pole pairs of the rotor.
In order to further increase the reliability of the determination of the direction of the rotor magnetization it is advantageous to provide at least one further sensor by means of which the partial flux of the magnetic flux of the stray field of the permanent magnetic rotor can be determined in a further measurement direction which is different from the first and the second measurement direction. Through this measure an error tolerance of the sensor arrangement can be achieved because the direction of the rotor magnetization can already be determined from two of the three sensor signals.
Furthermore, it is advantageous when each sensor comprises two sensor elements which are arranged so as to be displaced by 180xc2x0 in the direction of rotation of the rotor with respect to the desired axis of rotation of the rotor and which thus lie pair-wise oppositely with respect to the desired axis of rotation. With an arrangement of this kind systematic errors such as common mode disturbances, offsets and thermal drifts can be compensated.
From the point of view of the assembly technology and in regard to a particularly simple evaluation of the sensor signals it is advantageous to arrange the sensors and/or the sensor elements in such a manner that they measure the diametral, the radial or the tangential partial flux at their location.
In accordance with a preferred further development of the sensor arrangement at least two position sensors are furthermore provided for the determination of the radial position of the rotor. The stator of a bearing-free motor with a permanent magnetically excited rotor typically has a plurality of stator teeth which extend radially in the direction towards the rotor. The position sensors are then arranged in such a manner that the magnetic flux in the space between the rotor and the stator can be measured with them at two different measurement locations. With an arrangement of this kind, in addition to the direction of the rotor magnetization and the axial position of the rotor, its radial position in the stator can also be determined.
Preferably the position sensors are in each case arranged in a gap between two adjacent stator teeth because through this measure the magnetic control flux for the control of the rotor position which flows through the stator teeth is not also measured by the position sensors.
In order that a reliable determination of the radial position of the rotor for every rotor angle is also possible for those rotor magnetizations which have zero crossings, at least three position sensors are preferably provided which are arranged in such a manner that with them the magnetic flux in the space between the rotor and the stator can be determined at three different measurement locations, the position of which is such that for each rotational angular position of the rotor, that is, for each rotor angle, the angular position of at least two measurement locations is different from the angular position of the zero crossings of the rotor magnetization.
It is also advantageous for the position sensors when each position sensor comprises two sensor elements which are arranged so as to be displaced with respect to one another by 180xc2x0 in the direction of rotation of the rotor with respect to the desired axis of rotation of the rotor because the already mentioned systematic errors in the determination of the radial position of the rotor can also be thereby compensated.
The method in accordance with the invention for the operation of an electromagnetic rotary drive which is designed as a bearing-free motor with a magnetically journalled permanent magnetic rotor and a stator, in said method the direction of the rotor magnetization being determined with the help of at least two sensors for the determination of a magnetic flux, is characterized in that partial fluxes of that magnetic field which the stray field of the permanent magnetic rotor produces are used for the determination of the direction of the rotor magnetization.
For the previously mentioned reasons the following measures are also advantageous for the method in accordance with the invention:
the partial fluxes of the magnetic flux of the stray field are determined in a first and in a second measurement direction, said measurement directions in each case extending perpendicular with respect to the axial direction which is defined by the desired axis of rotation of the rotor;
the partial fluxes of the magnetic flux are determined outside the space which is located between the rotor and the stator;
the partial fluxes of the magnetic flux are in each case determined at the same axial distance from the rotor;
the difference signals of two sensor elements which in each case form a sensor and which are arranged with a displacement by 180xc2x0 with respect to one another in the direction of rotation of the rotor with respect to the desired axis of rotation of the rotor are used for the determination of the direction of the rotor magnetization;
in each case the diametral, the radial or the tangential partial flux is determined by means of the sensors and/or the sensor elements.
The direction of the rotor magnetization is preferably determined by a trigonometric analysis of the partial fluxes or through a comparison of the partial fluxes with an electronic look-up table.
A further advantageous measure consists in computationally compensating the stray fields which are produced by the stator. This can for example take place in such a manner that the stray field of the stator is first measured with the sensors in a kind of calibration measurement which is carried out without the rotor and the values which are determined therefrom are written into a memory, e.g. an EPROM, and are stored there. Then during the operation of the bearing-free motor (with the rotor) the measurement values which are measured by the sensors can be corrected with respect to the signals resulting from the stray field of the stator.
In accordance with a further aspect of the method in accordance with the invention the partial fluxes of the magnetic flux of the stray field of the rotor are used for the determination of the axial position of the rotor. For this the relationship between the signals of the sensors and the axial position of the rotor are preferably stored in an electronic look-up table, and the axial position of the rotor is determined during the operation of the rotary drive from the partial fluxes of the magnetic flux with the help of this electronic look-up table.
In accordance with a preferred further development of the method in accordance with the invention the magnetic flux in the space between the rotor and the stator is furthermore determined at two different measurement locations and the radial position of the rotor is determined using the direction of the rotor magnetization, the magnetic flux of which is determined at the measurement locations in the space between the rotor and the stator and the flux distribution in the air gap when the rotor is centered. As has already been mentioned the stator of a bearing-free motor with a permanent magnetically excited rotor typically has a plurality of stator teeth which extend radially in the direction towards the rotor. By the space between the rotor and the stator then both the air gap between the stator teeth and the rotor and the gaps between the stator teeth are meant.
As has previously been explained in connection with the sensor arrangement, in the method in accordance with the invention the magnetic flux in the space between the rotor and the stator is also preferably determined at least at three different measurement locations which are chosen in such a manner that for each rotational angular position of the rotor, that is, for each rotor angle, the angular position of at least two measurement locations is different from the angular position of the zero crossings of the rotor magnetization.
It is also advantageous for the method in accordance with the invention to additionally determine the magnetic flux in the space between the rotor and the stator at those locations which are displaced by 180xc2x0 with respect to the measurement locations when viewed in the direction of rotation of the rotor and to use them for the determination of the radial position of the rotor because systematic errors can be compensated through this.
The invention will be described in more detail in the following, both in relation to the aspects of the apparatus and of the technology of the method, with reference to exemplary embodiments and with reference to the drawings. In the drawings, parts which are identical or have equivalent functions are provided with the same reference symbol. The schematic drawings are not to scale.